The invention relates generally to an apparatus and a method for determining the freezing temperature of a liquid, and in particular the freezing temperature of a liquid in a liquid container of a motor vehicle.
Methods and measuring devices for use in the early detection of the required antifreeze agent concentration in a liquid dropping below a prescribed desired value in a window-cleaning system in modem motor vehicles, and thus in avoiding possible damage by frost are known in principle. Such methods and measuring devices are operated, for example, with the aid of a visual and/or audible warning display.
However, such methods and measuring devices have disadvantages in that the cleaning liquid and antifreeze agents are essentially metered as judged by eye, since it is most often the case that neither the liquid container of the window-cleaning system, nor the antifreeze agent container have a suitable scale. Consequently, the metering instructions are therefore frequently not followed. It also turns out to be disadvantageous in that over time the liquid has added to it a multiplicity of different additives, such as polyhydric alcohols or surfactants whose composition and interaction can not easily be determined.
In order, nevertheless, to save the vehicle owner with troublesome manual checking, it is also conceivable to equip a measuring device with a chemical sensor which can detect the composition of the liquid and thus permit a conclusion to be drawn concerning the lowering of the freezing point by means of a control unit. In practice, however, it has emerged that deposits form on the surface of such a sensor and influence the measurement result. A constant high measuring accuracy therefore requires the sensor to be cleaned at regular intervals. The advantage obtained over a manual measurement is thereby completely lost.
Therefore, there is a need for a method and device which reliably give information about the freezing point of the liquid, even when used over a lengthy period. Other needs will become apparent upon a further reading of the following description taken in conjunction with the drawings.
The above mentioned needs are addressed, according to the present invention by a device and a method for determining the freezing temperature of a liquid in a motor vehicle. The present invention cools a partial volume of the liquid and, in the process, the temperature and the change in aggregate state of the liquid in the partial volume are detected. It is thereby possible to determine the freezing temperature of the liquid without knowing the exact composition of the liquid. At the same time, the method can deliver measurement results which remain reliable over a long period since, by comparison with chemical sensors, the temperature sensor is relatively insensitive and, in particular, requires no regular cleaning or maintenance. Substantially more accurate measurement results can be achieved in this case, since the freezing temperature is measured directly, and therefore the conclusion is drawn from the composition of the liquid as to the freezing temperature, and thus also, at the same time, a possible source of error is eliminated.
One embodiment of the new process is that the freezing point of the liquid can be determined by detecting a physical change in the aggregate state, for example, by using a pressure measuring cell which can detect the expansion associated with freezing. A particularly advantageous embodiment of the invention is, by contrast, achieved by virtue of the fact that the heat of solidification released upon the change in the aggregate state of the liquid located in the partial volume is detected as a delayed temperature decrease. In order to determine the freezing temperature, in this case the liquid partial volume is uniformly cooled in a chamber connected to the main liquid chamber. As the change from the liquid to the solid phase begins, the heat of the change is released as the heat of solidification. This heat of solidification can be detected by a cooling element as a delayed decrease in temperaturexe2x80x94recorded, for example, by a temperature sensorxe2x80x94in conjunction with unchanged thermal dissipation. The freezing temperature is therefore determined with high accuracy from this temperature plateau, thus dispensing with the complete freezing of the liquid. By comparing the freezing temperature difference between the liquid and pure water, it is also possible to simultaneously deduce the concentration of additives lowering the freezing point.
Another advantage of the invention is that the invention detects changes in the electric admittance associated with the changes in the aggregate state of the liquid. The electric admittance or the electric resistance of the liquid is measured for this purpose. A central processing unit can be used to calculate a freezing temperature from a known temperature dependence of the admittance of the liquid. It is also conceivable to determine the freezing temperature by extrapolating representative values far above the freezing temperature.
Another advantage of the invention is that deviations in the calculation of the freezing temperature can be minimized by heating cooled liquid after determining the freezing temperature, and repeating the measuring process. Simultaneously, by heating the liquid beyond the level of the ambient temperature, the substances contained in the liquid are effectively mixed, thereby decreasing inaccuracies due to local differences in concentration.
It is also particularly useful to add an antifreeze agent, if appropriate, after the determination of the freezing temperature. By adding the antifreeze agent which lowers the freezing temperature, the freezing temperature can be set to a desired value without manual intervention. The antifreeze agent may be put into an auxiliary container connected to the liquid container, and can be fed to the liquid by means of an automatic valve. It is also conceivable to use the volumetric expansion upon freezing of the liquid for the purpose of delivering the antifreeze agent by, for example, using the volumetric expansion like a diaphragm pump to deliver the antifreeze agent.
Another advantage of the invention is that a signal for the motor vehicle occupant indicating that addition of antifreeze agent is required is triggered when the measured freezing temperature deviates from a desired value. The motor vehicle occupant is thereby informed about all relevant data, in order thus to be able to monitor the execution and the result of the determination of the freezing temperature, and take appropriate steps. In this case, it is also possible to determine the relationship between the determined freezing temperature of the liquid and the ambient temperature and, if appropriate, to trigger appropriate corresponding advice for the vehicle occupant.
The device, according to the present invention, determines the freezing temperature of a liquid, comprises of a chamber, enclosing a partial volume of the liquid, an opening in the chamber to permit an exchange of the liquid, a cooling element projecting into said chamber, and a sensor to determine the change in the aggregate state of the partial volume of liquid enclosed by the chamber and cooled by means of the cooling element.
It is thereby possible to reliably determine the freezing temperature of the liquid independently of the chemical composition. The liquid in the chamber is cooled in this case continuously by means of the cooling element until the measured temperature drops no further with continuing heat dissipation because of the heat of solidification being released. This temperature plateau simultaneously indicates the reaching of the freezing temperature, which can be detected by using the control unit to balance the dissipated heat with the measured temperature. Because only a partial volume of the liquid is cooled, while the remaining liquid volume is capable of further use, the small volume reduces the time and energy required to measure the freezing temperature.
Another particularly favorable configuration of the invention results when the device has a further sensor for detecting the aggregate state of the liquid. Such a sensor, designed as a pressure-measuring cell, for example, permits the freezing temperature to be determined accurately even if it is impossible to undertake reliable detection of the heat dissipated by the cooling element.
Also particularly simple is a development of the invention in which the further sensor is designed for determining the admittance of the liquid. As a result, the outlay associated with the freezing of the liquid can be reduced by calculating the freezing temperature in the case of a cooling of the liquid undertaken to a small extent, and of the change in the admittance measured in this case.
It is also particularly expedient in this case when the cooling element is a Peltier element. The thermal dissipation achieved by means of the Peltier element can be recalculated thereby without difficulty on the basis of the power consumption. At the same time, the Peltier element can also be used as a heating element in order in this way to be able to heat the liquid quickly after freezing, it also being possible at the same time to improve the mixing of the various constituents of the liquid.
It has proved to be particularly advantageous when the chamber enclosing a partial volume of liquid is arranged in the interior of the liquid container. Only a partial volume of the liquid is cooled, while the remaining liquid volume in the main container can be used further. The chamber may be arranged, for example, in the interior of the liquid container on a wall surface or bottom surface, and connected to the liquid container by means of a small opening. Furthermore, the device may be retroactively fitted without difficulty in the case of existing facilities and can be used to detect the temperature of a liquid in the liquid container even without being fixed. Another possibility of arranging the chamber is by connecting the chamber to the suction connection of the pump delivering the liquid.
It is particularly advantageous when liquid between the chamber and the liquid container may be exchanged by means of a valve. As a result, liquid is prevented from escaping, thus preventing warm liquid from flowing and allowing the temperature to be measured faster. Further, this configuration avoids measuring errors which can arise from partial freezing of individual constituents of the liquid.
The chamber may also be connected to the liquid container to simultaneously exchange the partial liquid volume inside the chamber each time the liquid container is filled with fresh liquid. Consequently, each filling of the liquid container with fresh liquid mixes the liquid inside the chamber. Thus, the contents of the partial liquid volume in the chamber substantially correspond to the liquid mixture in the liquid container such that local differences in concentration may be prevented. The chamber is fitted for this purpose in the region of the filling opening of the liquid container; thereby each time the liquid container is filled, the partial volume of liquid in the chamber is filled or emptied in accordance with the principle of a suction jet pump.
Another embodiment of the invention has the sensor of the device arranged inside an absorbent medium holding a partial volume of the liquid. The absorbent medium, designed as a sponge, for example, effectively prevents the cooled liquid from being exchanged with the remaining liquid outside the chamber. Consequently, only a small liquid volume needs to be cooled to determine freezing point. The absorbent medium can be used on the liquid container without structural changes. It is also possible to delimit a partial volume of the liquid by using a diaphragm.
The invention is suitable in principle for any liquids such as, for example, an engine coolant or a brake liquid. In the case of brake fluid, the invention may be applied in a similar way to determine the water content of the brake fluid. Additionally, the invention is particularly well-suited for measuring temperature of a cleaning liquid, in which the liquid is a liquid mixture with an antifreeze agent, because the invention allows the cleaning system to have any desired composition of the liquid mixture, and the device essentially does not require any kind of care or maintenance.